I am totally used to running a Debian system for my lighttpd server and MySQL. Would there be any benefits to running an Ubuntu 10.04 web and mysql server vs my Debian 5 web server any mysql servers? (I have two servers, one running http and the other running db._
My personal preference is Debian. The comparision is between Debian Stable and Ubuntu LTS, I presume. Ubuntu has a somewhat more formalized way (organised by the distro) to get commercial support.
Another consideration is the kernel version you'll be getting (stuck with).
Note that Debian is at the time of writing in the final stages of a new release (which you can pre-opt into today by installing squeeze -- also note that those final stages carry no deadline and may drag on).
Second note: shouldn't we be calling it Debian/Linux by now, the freebsd kernel thing is comming along quite nicely.
For the Ubuntu user perspective see http://ubuntu.stackexchange.com/questions/1471/is-it-better-to-use-debian-then-ubuntu-server-in-production.
Spoiler, it doesn't make much difference.
I would say if you run Debian stick to Debian.
If you wanted to pay for support you have the option to run Ubuntu and pay Canonical but if you are a Debian SysAdmin you probably don't want this.
Or you might need UbuntuOne - :).
For my perspective I run a few Ubuntu servers and they have been fine and I have not found fault with the stability.